The Aeronautical Telecommunications Network (ATN) is being progressively established around the world to provide for air-ground digital data communications. The ATN is dedicated to information exchange between aircraft and ground stations for activities such as Air Traffic Control (ATC), and Aeronautical Operational Communication (AOC). The ATN includes air-ground telecommunications facilities installed onboard aircraft, and ground stations equipped with air-ground telecommunications facilities. These air-ground telecommunications facilities providing a link between aircraft and ground stations constitute a subnetwork.
One such subnetwork is the Very High Frequency (VHF) Digital Link Mode 2 (VDLM2) subnetwork. In the near future, usage of the VDLM2 subnetwork will increase as VDLM2 was selected as the primary subnetwork for Controller Pilot Data Link Communications (CPDLC) in both the European Union and the United States. In addition, the VDLM2 subnetwork is being deployed in the Asia Pacific region. In order to meet the oncoming higher demand, new VDLM2 ground stations are being constructed by network operators such as Aeronautical Radio, Inc. (ARINC) and SociétéInternational de Télécommunications Aéronautiques (SITA). The need for optimal usage of the dense network of VDLM2 ground stations will grow dramatically in the future.
When using the VDLM2 subnetwork, there is a need to handover the communication link from one ground station to another. The algorithms currently in place determine when to handover, and which ground station is available for the handover. These algorithms work mostly with the strength of the ground station signal and with the trend of the signal strength, but are not always optimal because there are situations where more than one ground station may be suitable for handover. For example, all of the available ground stations may have a positive signal trend, but the one with the strongest signal may not be the best choice depending on the flight path of the aircraft. If a less than optimal ground station is selected, the number of handovers during aircraft flight will increase and the availability of the subnetwork will be diminished.